1. Field of the Invention
The present invention relates to silicon-on-insulator (SOI) substrates, and specifically to an SOI substrate using a transparent supporting substrate, a method for making the same, a liquid crystal panel, and an electronic device using the SOI substrate.
2. Description of Related Art
Since SOI technologies including formation of a silicon thin film on an insulating substrate and formation of a semiconductive device on the silicon thin film have advantages such as high speed response of devices, low electrical energy consumption, and high integration density, they have been widely studied.
One SOI technology is production of an SOI substrate including bonding single-crystal silicon substrates. The method, generally called a bonding method, includes bonding a single-crystal silicon substrate and a supporting substrate by means of hydrogen bonding, reinforcing the bonding strength by heat treatment, and forming a thin film single-crystal silicon layer by grinding, polishing or etching of the single-crystal silicon substrate. Since this method is capable of directly thinning the single-crystal silicon substrate, the resulting silicon thin film has high crystallinity and thus enables production of a high performance device.
Furthermore, applied bonding methods have been known, such as, for example, a method for doping hydrogen ions on a single-crystal silicon substrate, bonding it with a supporting substrate, and separating a thin film silicon layer from the hydrogen-doped region of the single-crystal silicon substrate by heat treatment (U.S. Pat. No. 5,374,564); and a method for epitaxially growing a single-crystal silicon layer on a silicon substrate with a porous surface, bonding it to a supporting substrate, removing the silicon substrate, and etching the porous silicon layer to form an epitaxial single crystal silicon thin film on the supporting substrate (Japanese Patent Application Laid-Open No. 4-346418). SOI substrates by bonding methods have been used in production of various devices, as well as general bulk semiconductive devices. An advantage of the SOI substrate not achieved by conventional bulk substrates is allowing the use of various materials as supporting substrates. That is, transparent quartz and glass substrates, in addition to general silicon substrates, can be used as supporting substrates. Formation of a single-crystal silicon thin film on a transparent substrate enables formation of devices requiring light transmissivity, for example, high-performance transistor devices using single crystal silicon in a transmissive liquid crystal display device.
In an SOI substrate including a transparent supporting substrate and a single-crystal silicon thin film bonded thereto, the single-crystal silicon layer is used as source and drain regions in transistor devices, such as metal oxide semiconductor field effect transistors (MOSFETs). When the substrate is transparent, light incident on the rear face of the substrate causes current leakage in the channel region of the MOSFET, and thus causes deterioration of device characteristics. (Herein the face of the substrate provided with the single-crystal silicon layer is called the front face, and the reverse face is called the rear face).
This will be described with reference to the drawings. FIG. 2 is a cross-sectional view of an SOI substrate provided with a transparent substrate produced conventionally. The SOI substrate has a configuration in which a single-crystal silicon layer 2 is bonded to a supporting substrate 1 with an oxide layer 3 therebetween. Since the oxide layer 3 generally transmits light, a conventional SOI substrate using a transparent material, such as quartz and glass, as a supporting substrate is not provided with a light shielding layer under the single-crystal silicon layer 2.
FIG. 3 is a cross-sectional view of a MOSFET produced using the conventional SOI substrate shown in FIG. 2. The oxide layer 3 is provided on the supporting substrate 1, and a source region 2b, a channel region 2a, and a drain region 2c of the MOSFET are formed by patterning of the single-crystal silicon layer. The single-crystal silicon layer is covered with a gate insulating film 2d formed by surface oxidation thereof. A gate electrode is provided on the gate insulating film 2d, and the single-crystal silicon layer and the gate electrode 6 of the MOSFET are covered with a first interlayer 7. The source region 2b and the drain region 2c are connected to a source line 9 and a drain line 8, respectively, through openings in the first interlayer 7. A second interlayer 10 is formed thereon, and an upper light shielding layer 11 is formed on the second interlayer 10. The upper light shielding layer 11 is formed of a nontransparent insulating material such as a polyimide resin or a metallic thin film such as aluminum. When light 12a is directly incident on the front face of the substrate, the upper light shielding layer 11 suppresses leakage of the light 12a to the channel region 2a of the MOSFET provided on the substrate. When light 12c is directly incident on the rear face of the substrate, leakage of the light to the channel region 2a of the MOSFET is not prevented. Light 12b reflected on the rear interface la of the substrate partly reaches the channel region 2a of the MOSFET and causes light leakage even if the light is incident on the front face of the substrate.
Since the conventional SOI substrate shown in FIG. 2 has no light shielding layer between the supporting substrate 1 and the single-crystal silicon layer 2, the channel region 2a of the MOSFET composed of a single-crystal silicon thin film using the SOI substrate is not shielded from the light 12c directly incident on the rear face of the substrate and the light 12b reflected on the rear face of the substrate. Thus, light leakage occurs in a MOSFET produced using an SOI substrate having the conventional configuration, resulting in a fundamental problem of deterioration of device characteristics. Accordingly, it is difficult to use a transparent SOI substrate for a device using light and the SOI substrate cannot be used generally.